In general, automation and precision control apparatuses use a speed reducer (or a power transmission device) to reduce and transfer a rotational speed of a high load transferred via a drive shaft of a power source such as an electric motor, a servo motor, etc.
Recently, as equipment becomes compact, a speed reducer is also required to be miniaturized while precisely operating.
A speed reducer according to the related art includes RV series of Vigo Drive™ developed and sold by Teijin Seiki.
The Teijin Seiki's RV series includes a plurality of pin gears and a plurality of 1-stage reduction gears at an output end side to secure, in an input shaft, an insertion space in an axial direction to insert a drive shaft of a power source, and transfers a reduced motion.
However, the Teijin Seiki's RV series speed reducer has the following problems.
First, since a plurality of pin gears are used, during calculation of a reduction ratio, the reduction ratio is frequently calculated to be a noninterminating decimal. Thus, when the speed reducer is repeatedly operated, errors are accumulated and thus a degree of precision of a speed reduction motion deteriorates remarkably.
Second, since the diameter of an output end side portion of the input shaft decreases in order to place the 1-stage reduction gear at the output end side, the speed reducer may be easily applied to a high reduction ratio. However, in order to apply the speed reducer to a low reduction ratio, separate components need to be added, and thus the speed reducer becomes large.
Third, since the 1-stage reduction gear is located at the output end, in order to prevent oil such as grease from leaking from a peripheral portion of the 1-stage reduction gear, a plurality of separate sealing components are provided on an outer circumferential surface of each 1-stage reduction gear, and thus the manufacturing costs and weight of the speed reducer are increased.
Fourth, as the 1-stage reduction gear are located at the output end side, while the size in a lengthwise direction of the insertion space in the input shaft to inert the drive shaft of the power source is guaranteed, the inner diameter of the insertion space may be quite limited due to the user of the pin gear.
Fifth, when the 1-stage reduction gear is installed at an input end side to overcome the above-described problems, the diameter of the insertion space to couple the drive shaft of the power source in a main body having a predetermined diameter is much decreased due to the 1-stage reduction gear. In particular, a speed reducible range is much limited.
Sixth, since the pin gear is employed, the number of pins to be accommodated in a predetermined main body is limited due to the diameter of a pin (about 2 mm), which may much limit the reduction range of the speed reducer.
As another speed reducer according to the related art, Korean Patent No. 1002422070000 (Prior Art 1) filed by the present applicant discloses an internal planetary reduction gear using a main crank shaft coupled to a drive shaft of a power source in an Oldham coupling method and receiving a rotational force from the power source, a plurality of planetary gears performing a speed reduction motion while performing a translational rotational motion according to the rotation of the auxiliary crank shaft, and a plurality of auxiliary crank shafts performing a translational rotational motion interconnected to the planetary gears.
However, in the speed reducer disclosed in the above-described Prior Art 1, since the auxiliary crank shafts are interconnected to the planetary gears that perform a translational rotational motion, the rotational motion of the main crank shaft and the rotational motion of the auxiliary crank shafts may not completely match each other due to processing, assembly, and deformation. In other words, the rotation motions of the auxiliary crank shafts are not restricted.
In this state, during a motion of the auxiliary crank shaft or when a load is applied from the outside, auxiliary crank shaft periodically vibrates in the rotational direction thereof, and thus continuity and precision in the power transfer may be reduced.
Accordingly, to solve the above problem, another invention was field by the present applicant and registered as Korean Patent No. 1003011200000 (Prior Art 2) which discloses an internal planetary reduction gear capable of miniaturization while maintaining continuity and precision in the power transfer with reliability.
However, in the speed reducer disclosed in the above-described Prior Art 2, since an input shaft is provided as one body, when a tooth shape on an outer circumferential surface of a rear end portion of the input shaft is greater than the space at the center of the speed reducer, the input shaft is unable to pass through a center space of a main body of the speed reducer so that the input shaft may not be installed at all or a separate auxiliary structure for the installation is needed. Consequently, an installation volume may be difficult to be reduced, and accordingly implementation of a relatively low reduction ratio may be restricted. Also, since it is known that a reduction range may be reduced, there is a demand for research and development of a new and improved internal planetary reduction gear which may solve the above problem.